Border ballads, like all traditional ballads, were traditionally sung unaccompanied. There may be a repeating motif, but there is no "chorus" as in most pop songs. The supernatural is a common theme in Border ballads, as are recountings of raids and battles.
Representative samples include "Thomas the Rhymer" (aka "True Thomas", Thomas of Erceldoune"), which opens in the English town of Erceldoune; and the very famous "Tam Lin".
The interested reader is referred to Minstrelry of the Scottish Border by Sir Walter Scott (Ivanhoe).